The present invention relates to a method for controlling knocking in multi-cylinder internal combustion engines having 2n cylinders, at least one of the cylinders being able to function as a guide cylinder for a cylinder designated to be the guided cylinder upon activation of a guide-cylinder function, having a cylinder counter, at least one knock sensor and an evaluation unit for evaluating the knock-sensor signals from the individual cylinders.
The evaluation of the knock-sensor signals is usually performed with the aid of individual knock-detection characteristics for each cylinder.
Knocking is understood as an uncontrolled form of combustion, which may lead to engine damage. For this reason, the ignition angle is normally always selected with a safety distance to the knock limit. On the other hand, it is known that an internal combustion engine should be operated as close as possible to the knock limit in order to achieve optimum torque. Thus, keeping the ignition angle at a safety distance to the knock limit results an increased fuel consumption. The knock limit depends on a variety of factors during the operation of an internal combustion engine, such as fuel quality, engine state and environmental conditions. In order to be able to select the safety distance as small as possible, that is, to be able to bring the ignition angle as close as possible to the instantaneous knock limit, the respective instantaneous knock limit must first be determined in each case. It is standard practice to use knock sensors for this purpose, such as structure-borne noise sensors or combustion chamber pressure-sensors.
The knock-sensor signal is individually evaluated for each cylinder. Assigning the instantaneous knock-sensor signal to a particular cylinder, which is required for this purpose, is performed with the aid of the cylinder counter within the framework of a phase detection in which the states of the individual cylinders during operation of the internal combustion engines are detected. Under certain conditions, if the phase detector fails, it no longer possible to determine whether the knock-sensor signal which is to be assigned to the cylinder corresponding to the cylinder-counter reading is being evaluated or that of the cylinder running at a 360 degree offset. This case is called knock-control (KC) emergency operation.
In practice, given KC emergency operation, the knock-detection threshold having the most sensitive setting of all cylinders is selected and used as the basis of the ignition-timing control for all cylinders. The selection of the corresponding knock-detection characteristic must then be checked whenever the knock-detection threshold has been changed, so that undetected knocks and, thus, engine damage may be avoided.
Independently of KC emergency operation, it may occur that, starting with a specific rotational speed, a reliable knock detection is no longer possible for individual cylinders. In this case, the affected cylinder is designated as a cylinder to be guided, and the so-called guide-cylinder function is activated. In this way, one or even a plurality of guide cylinders is/are assigned to the cylinder to be guided. The cylinder to be guided is then operated at the ignition timing retard of the guide cylinder(s) and, if appropriate, has an additional offset applied. If it is not possible to assign a guide cylinder to an affected cylinder, the latest ignition angle of the remaining cylinders is used for this cylinder. In this case, the ignition angle may have an additional offset applied as well.
In practice, it may occur that the guide-cylinder function is activated in KC emergency operation. In this case, the so-called safety-ignition retard is triggered in which an ignition angle is set which has been predefined for all cylinders. The resultant setting of the ignition angles to retard causes a high loss in torque.
The present invention provides a method for knock control in KC emergency operation and activated guide-cylinder function which make it possible to avoid torque loss and to maintain the functionality desired by the guide-cylinder function even in KC emergency operation, while keeping the application cost low.
According to the present invention, this is achieved in that in KC-emergency operation and at activated guide-cylinder function
cylinder pairs are formed from those cylinders running in each instance at an offset of 360 degrees;
each twin pair having at least one cylinder to be guided being assigned at least one of the available guide cylinders, so that the ignition timing retard of the associated guide cylinder forms the basis of the ignition timing retard of the cylinder pair to be guided;
and a safety retard timing in which an ignition angle is set that has been predefined for all cylinders is triggered only when no sufficient number of guide cylinders is available.
According to the present invention, a knock control or an ignition timing control in KC emergency operation and a knock control or ignition-timing control with guide-cylinder function are interlinked in such a way that the ignition timing control is performed as a function of the state of the respective cylinder. This is because the knock control according to the present invention takes into account both whether a cylinder is affected by the unavailability of phase detection and also whether a reliable knock detection is still possible for a cylinder at the given rotational speed. For this purpose, according to the present invention, the cylinders of the internal combustion engines are combined into cylinder pairs. The ignition angles of the cylinders of a cylinder pair to be guided are uniformly controlled in KC emergency operation. Compared to a safety timing retard for all cylinders, the forming of cylinder pairs according to the present invention and the pair-wise control according to the present invention allows earlier ignition angles for at least some of the cylinders of the internal combustion engine, so that a torque gain is able to be achieved.
If a plurality of guide cylinders is assigned to the cylinder pair to be guided, the greatest torque gain may be achieved by ascertaining in each case the minimum of the ignition timing retards of all guide cylinders of the cylinder pair to be guided and this minimum of the ignition timing retards forming the basis for the ignition timing retard of the cylinder pair to be guided.
For reasons of application safety, it is advantageous if the cylinder pair to be guided is operated at the ignition timing retard of the guide cylinder or the minimum of the ignition timing retards of all the assigned guide cylinders plus an applicable offset.